The traders of DVDs containing digital contents such as movies are on their guard against the distribution or sale of pirated discs produced by unauthorized parties. Such pirated discs are produced with the use of two driving apparatuses. More specifically, a DVD containing a digital content is inserted into one (first) driving apparatus, and an optical disc is inserted into the other (second) driving apparatus. While the first driving apparatus obtains a read signal by reading the digital content from the DVD and converts the read signal into digital data, the second driving apparatus generates a write signal in accordance with the digital data, and writes a copy of the digital content onto the optical disc in accordance with the write signal.
The above operation is performed for each track on the whole (entire) data recording area of the DVD while the two discs are rotated in synchronization with each other by the spindle motors of the two driving apparatuses. In this way, a pirated disc containing a complete copy of the digital content in the DVD is produced. Typically, digital contents are recorded on DVDs after being encrypted by a standardized technique called a contents encryption method. It is thought that decrypting such digital contents is difficult. This contents encryption method, however, is invalid for the above-introduced pirated-disc producing technique since the digital contents, together with the encryption key, are copied as they are encrypted. If pirated discs are produced in an organized manner to allow thousands of pirated discs to appear on the market, copyright owners of digital contents will be damaged heavily.
Another technique for preventing the production of pirated discs superposes an encryption key by using jitters that appear on the read signal obtained from DVDs. General driving apparatuses convert read signals into digital data after correcting jitters by using a Phase-Locked Loop (PLL). Accordingly, if digital contents are copied by the above-mentioned pirated-disc producing technique using two driving apparatuses, the copies cannot be reproduced since the copied discs lack encryption keys.
However, when a DVD containing a digital content recorded by the other technique is reproduced, natural jitters appear on the read signal, as well as the jitters superposed as the encryption key. Such jitters may cause erroneous bits or bit shifts to prevent proper reproduction of the optical disc. This is especially the case for conventional reproduction apparatuses that cannot recognize jitters as an encryption key.
It also possible that the natural jitters are recognized as part of the encryption key to cause erroneous bits of the encryption key. When the encryption key is recognized erroneously due to this, the digital contents recorded on the DVD cannot be decoded. This damages the reliability of DVD and the reproduction apparatus.